1. Field of the Invention
The present invention relates to a gas sensor that can be used to control combustion in an internal combustion engine, such as an engine for a vehicle.
2. Description of the Related Art
Conventionally, there is a gas sensor that detects levels of various types of gas components within the exhaust gas.
One example of such gas sensors is shown in Japanese Patent Laid-open Publication No. 2002-82085. In this publication, as shown in FIG. 12, a gas sensor is installed in the exhaust system of an internal combustion engine, such as an engine for a vehicle.
Specifically, as shown in FIG. 12, the gas sensor 9 includes a sensing element 92, a housing 93, and an atmosphere-side cover 94. The sensing element 92 detects a particular gas concentration within the gas to be measured. The sensing element 92 is inserted into the housing 93 and held therein. The atmosphere-side cover 94 is fixed onto the base-end side of the housing 93. On the tip-end side of the housing 93, a cover 97 serving as a gas-being-measured-side cover is secured to the housing 93. This cover 97 is formed as a double-structure cover composed of an inner cover 971 and an outer cover 972. Gas introducing openings 973 are formed through both inner and outer covers 971 and 972. Hence, a gas 974 to be measured is introduced into the inside of the inner cover 71 through the openings 973.
As shown in FIG. 12, the atmosphere-side cover 94 includes a large diameter section 941, a small diameter section 942, and a stepped section 943. The large diameter section 941 is fixed onto the housing 93. The small diameter section 942 is mounted on the base-end side of the atmosphere-side cover 94. The stepped section 943 is formed to connect the large diameter section 941 and the small diameter section 942.
The base-end side of the atmosphere-side cover 94 is covered by a filter cover 96. In addition, as shown in the diagram, a waterproof ventilation filter 95 is sandwiched between the atmosphere-side cover 94 and the filter cover 96. The ventilation filter 95 is fastened by fastening sections 960 in a caulking manner, provided in two locations in the axis direction. As shown in FIG. 12, the ventilation filter 95 is positioned so that the tip-end section of the ventilation filter 95 is in contact with the stepped section 943.
Air-introducing openings 944 are formed through both covers 94 and 96, so that atmosphere 945 is introduced into the inside of the atmosphere-side cover 94 through the openings 944. The atmosphere 945 serves as a reference gas.
As is known, the sensing element 92 has the capability of sensing the concentration of a particular (i.e., designated) component of a gas to be measured. A sensed signal from the sensing element 92 is supplied to an external unit via lead portions 921, connectors 922, and lead wires 923.
When the gas sensor 9 is in use, the temperature of the tip-end side of the gas sensor 9 becomes high. The heat is transmitted to the base-end side, and the temperature of the ventilation filter 85 also becomes high. Therefore, the ventilation filter 95 becomes deteriorated by the heat, through repeated use of the gas sensor 9 in the exhaust system of the internal combustion engine. There is risk of the deterioration of the waterproof properties of the ventilation filter 95.
As a result, it is necessary for the ventilation filter 95 to be placed as close to the base end of the gas sensor 9 as possible. Further provision of a new stepped section on the base-end side of the stepped section 943 and disposition of the ventilation filter 95 near the base end of the gas sensor 9 can be considered. However, a narrow crevice is easily formed between the atmosphere-side cover 94 and the filter cover 96 when the stepped section is increased. When moisture or the like leaks into the crevice, it is difficult to drain the moisture or the like. Therefore, there is a risk of crevice corrosion caused by the moisture or the like.